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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Chapter 13.  Input and Output</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="std_contents.html" title="Part II.  Standard Contents" /><link rel="prev" href="numerics_and_c.html" title="Interacting with C" /><link rel="next" href="streambufs.html" title="Stream Buffers" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 13. 
  Input and Output
  
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    Standard Contents
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  Input and Output
  <a id="id-1.3.4.11.1.1.1" class="indexterm"></a>
</h2></div></div></div><div class="toc"><p><strong>Table of Contents</strong></p><dl class="toc"><dt><span class="section"><a href="io.html#std.io.objects">Iostream Objects</a></span></dt><dt><span class="section"><a href="streambufs.html">Stream Buffers</a></span></dt><dd><dl><dt><span class="section"><a href="streambufs.html#io.streambuf.derived">Derived streambuf Classes</a></span></dt><dt><span class="section"><a href="streambufs.html#io.streambuf.buffering">Buffering</a></span></dt></dl></dd><dt><span class="section"><a href="stringstreams.html">Memory Based Streams</a></span></dt><dd><dl><dt><span class="section"><a href="stringstreams.html#std.io.memstreams.compat">Compatibility With strstream</a></span></dt></dl></dd><dt><span class="section"><a href="fstreams.html">File Based Streams</a></span></dt><dd><dl><dt><span class="section"><a href="fstreams.html#std.io.filestreams.copying_a_file">Copying a File</a></span></dt><dt><span class="section"><a href="fstreams.html#std.io.filestreams.binary">Binary Input and Output</a></span></dt></dl></dd><dt><span class="section"><a href="io_and_c.html">Interacting with C</a></span></dt><dd><dl><dt><span class="section"><a href="io_and_c.html#std.io.c.FILE">Using FILE* and file descriptors</a></span></dt><dt><span class="section"><a href="io_and_c.html#std.io.c.sync">Performance</a></span></dt></dl></dd></dl></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="std.io.objects"></a>Iostream Objects</h2></div></div></div><p>To minimize the time you have to wait on the compiler, it's good to
      only include the headers you really need.  Many people simply include
      <code class="filename">&lt;iostream&gt;</code> when they don't
      need to -- and that can <span class="emphasis"><em>penalize your runtime as well.</em></span>
      Here are some tips on which header to use
      for which situations, starting with the simplest.
   </p><p><span class="emphasis"><em><code class="filename">&lt;iosfwd&gt;</code></em></span>
      should be included whenever you simply need the <span class="emphasis"><em>name</em></span>
      of an I/O-related class, such as "<code class="classname">ofstream</code>" or
      "<code class="classname">basic_streambuf</code>".
      Like the name implies, these are forward declarations.
      (A word to all you fellow old school programmers:
      trying to forward declare classes like "<code class="code">class istream;</code>"
      won't work.
      Look in the <code class="filename">&lt;iosfwd&gt;</code> header
      if you'd like to know why.)  For example,
   </p><pre class="programlisting">
    #include &lt;iosfwd&gt;

    class MyClass
    {
	....
	std::ifstream&amp;   input_file;
    };

    extern std::ostream&amp; operator&lt;&lt; (std::ostream&amp;, MyClass&amp;);
   </pre><p><span class="emphasis"><em><code class="filename">&lt;ios&gt;</code></em></span>
      declares the base classes for the entire I/O stream hierarchy,
      <code class="classname">std::ios_base</code> and <code class="classname">std::basic_ios&lt;charT&gt;</code>,
      the counting types <span class="type">std::streamoff</span> and <span class="type">std::streamsize</span>,
      the file positioning type <span class="type">std::fpos</span>,
      and the various manipulators like <code class="function">std::hex</code>,
      <code class="function">std::fixed</code>, <code class="function">std::noshowbase</code>,
      and so forth.
   </p><p>The <code class="classname">ios_base</code> class is what holds the format
      flags, the state flags, and the functions which change them
      (<code class="function">setf()</code>, <code class="function">width()</code>,
      <code class="function">precision()</code>, etc).
      You can also store extra data and register callback functions
      through <code class="classname">ios_base</code>, but that has been historically
      underused.  Anything
      which doesn't depend on the type of characters stored is consolidated
      here.
   </p><p>The class template <code class="classname">basic_ios</code> is the highest
      class template in the
      hierarchy; it is the first one depending on the character type, and
      holds all general state associated with that type:  the pointer to the
      polymorphic stream buffer, the facet information, etc.
   </p><p><span class="emphasis"><em><code class="filename">&lt;streambuf&gt;</code></em></span>
      declares the class template <code class="classname">basic_streambuf</code>, and
      two standard instantiations, <span class="type">streambuf</span> and
      <span class="type">wstreambuf</span>.  If you need to work with the vastly useful and
      capable stream buffer classes, e.g., to create a new form of storage
      transport, this header is the one to include.
   </p><p><span class="emphasis"><em><code class="filename">&lt;istream&gt;</code></em></span>
       and <span class="emphasis"><em><code class="filename">&lt;ostream&gt;</code></em></span>
       are the headers to include when you are using the overloaded
      <code class="code">&gt;&gt;</code> and <code class="code">&lt;&lt;</code> operators,
      or any of the other abstract stream formatting functions.
      For example,
   </p><pre class="programlisting">
    #include &lt;istream&gt;

    std::ostream&amp; operator&lt;&lt; (std::ostream&amp; os, MyClass&amp; c)
    {
       return os &lt;&lt; c.data1() &lt;&lt; c.data2();
    }
   </pre><p>The <span class="type">std::istream</span> and <span class="type">std::ostream</span> classes
      are the abstract parents of
      the various concrete implementations.  If you are only using the
      interfaces, then you only need to use the appropriate interface header.
   </p><p><span class="emphasis"><em><code class="filename">&lt;iomanip&gt;</code></em></span>
      provides "extractors and inserters that alter information maintained by
      class <code class="classname">ios_base</code> and its derived classes,"
      such as <code class="function">std::setprecision</code> and
      <code class="function">std::setw</code>.  If you need
      to write expressions like <code class="code">os &lt;&lt; setw(3);</code> or
      <code class="code">is &gt;&gt; setbase(8);</code>, you must include
      <code class="filename">&lt;iomanip&gt;</code>.
   </p><p><span class="emphasis"><em><code class="filename">&lt;sstream&gt;</code></em></span>
      and <span class="emphasis"><em><code class="filename">&lt;fstream&gt;</code></em></span>
      declare the six stringstream and fstream classes.  As they are the
      standard concrete descendants of <span class="type">istream</span> and <span class="type">ostream</span>,
      you will already know about them.
   </p><p>Finally, <span class="emphasis"><em><code class="filename">&lt;iostream&gt;</code></em></span>
      provides the eight standard global objects
      (<code class="code">cin</code>, <code class="code">cout</code>, etc).  To do this correctly, this
      header also provides the contents of the
      <code class="filename">&lt;istream&gt;</code> and
      <code class="filename">&lt;ostream&gt;</code>
      headers, but nothing else.  The contents of this header look like:
   </p><pre class="programlisting">
    #include &lt;ostream&gt;
    #include &lt;istream&gt;

    namespace std
    {
	extern istream cin;
	extern ostream cout;
	....

	// this is explained below
	<span class="emphasis"><em>static ios_base::Init __foo;</em></span>    // not its real name
    }
   </pre><p>Now, the runtime penalty mentioned previously:  the global objects
      must be initialized before any of your own code uses them; this is
      guaranteed by the standard.  Like any other global object, they must
      be initialized once and only once.  This is typically done with a
      construct like the one above, and the nested class
      <code class="classname">ios_base::Init</code> is
      specified in the standard for just this reason.
   </p><p>How does it work?  Because the header is included before any of your
      code, the <span class="emphasis"><em>__foo</em></span> object is constructed before any of
      your objects.  (Global objects are built in the order in which they
      are declared, and destroyed in reverse order.)  The first time the
      constructor runs, the eight stream objects are set up.
   </p><p>The <code class="code">static</code> keyword means that each object file compiled
      from a source file containing
      <code class="filename">&lt;iostream&gt;</code> will have its own
      private copy of <span class="emphasis"><em>__foo</em></span>.  There is no specified order
      of construction across object files (it's one of those pesky NP complete
      problems that make life so interesting), so one copy in each object
      file means that the stream objects are guaranteed to be set up before
      any of your code which uses them could run, thereby meeting the
      requirements of the standard.
   </p><p>The penalty, of course, is that after the first copy of
      <span class="emphasis"><em>__foo</em></span> is constructed, all the others are just wasted
      processor time.  The time spent is merely for an increment-and-test
      inside a function call, but over several dozen or hundreds of object
      files, that time can add up.  (It's not in a tight loop, either.)
   </p><p>The lesson?  Only include
      <code class="filename">&lt;iostream&gt;</code> when you need
      to use one of
      the standard objects in that source file; you'll pay less startup
      time.  Only include the header files you need to in general; your
      compile times will go down when there's less parsing work to do.
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